1. Field of the Invention
The present invention relates to a driving circuit, and more particularly, to a light source driving circuit for driving light emitting components.
2. Description of Related Art
Light source driving circuits have a wide application, which are used as the backlight driving circuits of liquid crystal displays or light emitting diode (LED) driving circuits of handheld mobile devices. In addition, light source driving circuits have a high potential for development. The light source driving circuit has many advantages that can be continuously improved or enhanced, such as enhancing power energy conversion efficiency and improving stability.
FIG. 1 shows the architecture of a conventional light source driving circuit for driving a (string of) light emitting diode (LED) 106. The circuit comprises a DC-DC power supply voltage converter 101, an output voltage feedback circuit 102, an output capacitor 103, a timing control circuit 104, and a dimming circuit 105. The dimming circuit 105 is coupled between the cathode of the LED string 106 and the common level GND and receives the pulse width signal PWM output by the timing control circuit 104. In addition, the output voltage feedback circuit 102 is implemented by connecting two resistors in series.
The power supply voltage converter 101 provides a DC power supply voltage to the LED string 106, and stabilizes the output voltage through the feedback signal fed back from the output voltage feedback circuit 102. The timing control circuit 104 determines the width of the output pulse width signal PWM according to the brightness set by a user. The dimming circuit 105 is turned on when the pulse width signal PWM is at a high level, and is turned off when the pulse width signal PWM is at a low level. Through adjusting the width of the pulse width signal PWM, the on/off state of the dimming circuit 105 is controlled, and further the LED string 106 is controlled to allow current passing through or not. As such, when the brightness is set higher, the time of the pulse width signal PWM being at the high level is longer, the time of the dimming circuit 105 being at the on-state is long, and the brightness of the LED string 106 is brighter. On the contrary, when the brightness is set lower, the time of the pulse width signal PWM being at the high level is shorter, the time of the dimming circuit 105 being at the on-state is shorter, and the brightness of the LED string 106 is dimmer.
However, when the pulse width signal PWM is at a low level, the dimming circuit 105 is not turned on, and at this time, all the voltage drop of the output voltage supplied by the power supply voltage converter 101 falls on the dimming circuit 105. When the LED string 106 has more LEDs connected in series, the output voltage supplied by the power supply voltage converter 101 becomes extremely high. When the withstand voltage of the dimming circuit 105 is insufficient, the dimming circuit 105 may be damaged by the output voltage supplied by the power supply voltage converter 101. Further, a dimming circuit 105 with higher withstand voltage must be employed, and the dimming circuit 105 with higher withstand voltage has a larger volume and is more expensive, which causes an increase in the volume and the cost of the integrated circuit, and thus reducing competitiveness of the product in the market.